A variety of surface flaw inspection techniques, in which a flaw in a surface of an object to be inspected, such as a surface of a thin steel sheet, is optically detected by irradiating the surface with light and analyzing light reflected from the surface, have been proposed.
For example, Japanese Patent Application Laid-Open (JP-A) No. 58-204353 proposes a process for detecting a flaw in a surface of a metal body, wherein a surface of an object to be inspected is irradiated with light, and specular light and diffused light reflected from the surface are detected by cameras. In this surface flaw detection process, light is made incident on the surface of the object to be inspected at an angle of 35° to 75°, and light reflected from the surface is received by two cameras, which are disposed in a direction in which specular light advances and a direction which is within 20° from the direction in which the incident light or the specular light advances. Subsequently, signals generated by the two cameras at the time of receiving light are compared with each other by, for example, obtaining a logical sum thereof. Only when the cameras detect an abnormal value at the same time, a portion having the abnormal value is regarded as a flaw. In this way, the surface flaw detection process which is not affected by noise is realized.
Further, JP-A No. 60-228943 proposes a process for inspecting a flaw in a surface of an object to be inspected by receiving backscattering light from the test substance. In this flaw inspection process, a flaw in a surface of a stainless steel sheet is detected by making light incident onto the stainless steel sheet at a large incidence angle and detecting light reflecting back, namely, backscattering light.
Moreover, JP-A No. 8-178867 proposes an apparatus for detecting a flaw in hot-rolled flat steel by detecting a plurality of reflected backscattering light beams. This hot-rolled flat steel flaw detection apparatus detects a scratch on the hot-rolled flat steel. In this flaw detection apparatus, the angle of a slope of a scratch is 10 to 40°, and plural cameras are disposed in directions in which reflected backscattering light beams advance, so as to receive all the specular light reflected from the slope of the scratch having an angle within the above range.
However, the purpose of the measuring techniques proposed above is to detect a remarkably irregular flaw. It has been difficult to reliably detect a flaw which is formed by adhesion on copper foil and not remarkably irregular.
For example, as for the flaw detection process disclosed in JP-A No. 58-204353, the two cameras for receiving specular light and scattered light are provided so that the influence of noise is removed by obtaining a logical sum of the detection signals generated by the cameras. Thus, the process can be used to detect remarkably irregular flaws, namely, flaws in the surface which are cracked, dug up or turned up because signals of these flaws can be picked up by both the cameras. However, it is impossible to reliably detect flaws which are not so remarkably irregular that only one of the cameras can pick up signals thereof.
Further, the surface condition inspection process disclosed in JP-A No. 60-228943 is intended to detect raised flaws, which are conspicuous on the stainless steel sheet having small surface roughness. Thus, this process cannot be used for defects which do not have conspicuously raised portions.
Furthermore, the apparatus for detecting a flaw in hot-rolled flat steel disclosed in JP-A No. 8-178867 is intended to detect scratches, and the detection is based on the capture of specular light reflected from the slope of a scratch. Thus, reflected backscattering light of some of the defects which are not remarkably irregular may not be captured by the apparatus.
In view of the above drawbacks, an object of the present invention is to provide a defect inspection apparatus and a defect inspection process for detecting with high accuracy a defect of an object to be inspected. Particularly, an object of the present invention is to provide a copper foil inspection apparatus and a copper foil inspection process for detecting with high accuracy a defective copper portion of a surface of copper foil, which portion is likely to remain after etching.